In a large bowl, combine popcorn, cereal, M&M's and pretzels; set aside.
In a large microwave-safe bowl, combine the brown sugar, butter, corn syrup and flour. Microwave, uncovered, on high for 3-4 minutes or until a candy thermometer reads 244°-248° (firm-ball stage), stirring after each minute.
Immediately pour over popcorn mixture; toss to coat. Press into a buttered 15x10x1-in. baking pan. Cool completely. Cut into bars.